exportHistory method
Exports notification history as JSON or CSV.
Implementation
String exportHistory({DateTime? since, String format = 'json'}) {
final filtered = since != null
? _notifications.where((n) => n.timestamp.isAfter(since)).toList()
: List<AppNotification>.from(_notifications);
if (format == 'csv') {
return _exportCsv(filtered);
}
return _exportJson(filtered);
}